Agitating elements are used for mixing liquids in a vessel. Some of these conventional agitating elements include agitating blades of constant width while others include blades that taper off toward one end. In addition to these agitating elements, propeller agitators are also used to mix liquids in a container. These propellers are usually symmetrical and have trailing agitating blades.
FIG. 7 shows a one-half cross sectional view of one example of such a conventional agitating device. A rotating shaft 62 is placed in a vessel 60, and a prior art agitating element 61 is connected to the rotating shaft via a hub 12. When the conventional agitating element 61 is rotated to agitate materials in a vessel 60, the agitating element 61 tends to generate short circuited fluid flows 64 as indicated by the arrows near the agitating element 61. An area of the short circuited flows 64 is essentially stagnant and considered as a dead zone. Because of these local short circuited flows, the material near the agitating element 61 is circulated only in the vicinity of the agitating element 61 and does not travel over a significant vertical distance. As a result, the material near the bottom and near the top of the vessel 60 are not mixed together. The above described separation problem becomes more serious when material is to be mixed using a conventional agitating element in a deep vessel.
A prior attempt to solve the above described problem included a control of axial velocities generated by an agitating element. Although the prior attempt distributed axial velocities more or less uniformly on the discharge surface of the agitating element, the short circuited flow was not effectively controlled. Other attempts included agitating elements that were manufactured based upon the lifting surface theory. However, these prior art agitating elements also failed to suppress a short circuit flow near the agitating element, and their efficiency for mixing material did not significantly improve.